This invention relates to furnaces and their operation, and in particular, to furnaces including a plurality of combustion chambers or firing ports which receive heated combustion air from a common distributing chamber or plenum, for combining with fuel to produce a combustion flame. More particularly, the invention relates to controlling the apportionment of combustion air among such chambers or ports in, e.g. a multiport regenerative furnace.
More particularly, the invention relates to controlling the amount of combustion air delivered to any individual firing port from a common combustion air distribution chamber or plenum, e.g. in a multiport regenerative furnace having a nonpartitioned plenum. Such furnaces generally include a plurality of firing ports on each side of the furnace chamber, with ports on one side aligned with, in opposing relation to, ports on the other side. During the firing phase, combustion air passes into a lower plenum, through the regenerator, where it is heated, through the upper plenum to the firing ports where it is combined with fuel for producing a combustion flame for heating material in the furnace chamber. Exhaust gases from each port pass through its opposing port and down through the opposite regenerator for heating the regenerator packing. In other words, while the exhaust phase firing port is receiving exhaust gases, its regenerator is absorbing heat from the exhaust gases and the other firing phase firing port is receiving heated combustion air through its regenerator. The side of the furnace receiving fuel and heated combustion air is periodically reversed so that each side alternately participates in an exhaust phase and a firing phase.
As used herein, combustion air is not limited to any particular combination of gases or proportions thereof, but is used for simplicity to refer to any gas which, when combined with fuel produces a combustible mixture.
The problem of uneven combustion air distribution within the regenerator of a furnace having a gas distributing chamber or plenum atop a regenerator bed is discussed in U.S. Pat. No. 4,375,236 to Tsai, the teachings of which are hereby incorporated by reference. As taught therein, air jets can be utilized to affect a more uniform distribution within regenerator by influencing distribution in the plenum. However, in a multiport furnace, e.g. a glass melting furnace, it is common to have individual ports operating at different firing rates in order to optimize furnace efficiency. More particularly, furnace efficiency depends upon proper distribution of energy input among the firing ports in order to provide an appropriate amount of energy for the portion of the furnace underlying the port. In general, a change in furnace operating conditions such as throughput, melt composition or furnace upset requires a redistribution of energy inputs among the ports to maintain furnace efficiency within an acceptable range. This redistribution is generally accomplished for a given total fuel input by apportioning the fuel among the ports in a nonuniform manner.
The nonuniform fuel distribution requires a correspondingly nonuniform distribution of combustion air among the ports. Too much combustion air relative to the amount of fuel in a particular firing port may be undesirable because the excess heated air is not utilized for combustion. Too little combustion air results in wasted, non-combusted fuel. Although it is possible to increase total combustion air input, e.g. by adjustment of the blower, and thus effect an increase in air to ports where needed this also increases air to ports which may already have the desirable amount of air resulting in less than desired overall efficiency. In other words, for any given total fuel input, there is an optimal amount of total combustion air needed for overall furnace efficiency. At the same time, for any individual port, there is also an optimum amount of combustion air for its fuel input. Consequently, disproportionate fuel input among the ports requires a correspondingly disproportionate combustion air input to affect the desired fuel-air mixture and hence optimize efficiency. It would therefore be desirable to selectively increase or decrease air in only these ports where it is needed while maintaining the proper amount of total combustion air input to the furnace relative to the total fuel input.
In the past, control of the combustion air distribution in individual firing ports has been accomplished primarily by using barriers, e.g. damper tiles made of refractory material, inserted in various locations within the firing port to partially obstruct the flow of combustion air through the port. With this technique, total combustion air can be increased, e.g., by increasing blower output, and then dampers are used in particular ports where no increase in combustion air is desired. The use of refractory barriers or dampers, however, has several drawbacks. The refractory dampers are expensive and begin to deteriorate fairly quickly in the harsh firing port atmosphere creating accumulating debris on the port floor. This debris further obstructs flow of combustion air in an uncontrolled manner and is difficult to remove. Attempts to rake the debris from the port can result in moving the debris into the regenerators causing clogging and a decrease in flow rate through the regenerator. Furthermore, although dampers can be useful to decrease combustion air flow through a port, they are not suitable for increasing combustion air. In addition, dampers have the substantial drawback of providing only imprecise control limited by the size and shape of refractory blocks or tiles which can be conveniently inserted into the firing port at fixed locations to obstruct flow.
It would therefore be advantageous to have a method of selectively controlling combustion air in a firing port which does not have the limitations of presently available techniques.